Tertiary Geology of the Area Between Milford, Utah and Pioche, Nevada

Abstract After a long period of tectonic quiesence and erosion following the Sevier orogeny, volcanism began about 34 Ma in the area between Pioche, Nevada and Milford, Utah when local rhyolitic ash-flows and andesitic lavas were erupted. From 33 to 26 Ma the Needles Range magma system astride the Nevada-Utah state-line broadcast about 104 km3 of dacite and rhyolite ash-flows over an area of 20,000 km2; subordinate andesite lava flows erupted from many local vents above the magma chamber, chiefly during the earlier activity of the system. Recurrent ash-flow eruptions at intervals of about one million years define three compositional cycles, each began with emplacement of hundreds of km3 of compositionally zoned, crystal-poor rhyolite in multiple cooling units and culminated with eruption of thousands of km3 of apparently uniform crystal-rich dacite. A tight cluster of partially superposed, more or less southward time transgressive calderas formed during these voluminous eruptions. After the Oligocene, magmatism between Pioche and Milford shifted to much smaller eruptions from numerous vents without caldera formation and compositions became increasingly bimodal. From 23 to 18 Ma trachyandesite lava flows, rhyolite lava and ash flows, and more local dacite and latite lava flows were extruded. Northeast-striking, high-angle faulting accompanied this magmatic episode. From 13 to 12 Ma, high-silica topaz rhyolite lava flows and potassic basaltic lava flows were extruded.